1. Field of the Invention
This invention relates generally to the field of photon diagnostics including displacement or Doppler interferometry, and more particularly, to a portable and durable apparatus which incorporates the use of a remote sensing probe in fiberoptic communication with an interferometer or spectrometer. Also disclosed here are several applications for the apparatus including optically measuring high velocities and analyzing plasma/emission spectral characteristics. Applications for the apparatus include analyzing shock wave effects and detecting and destroying tumors in living tissue.
2. Description of the Related Art
Interferometric analysis concerns a group of techniques whereby a beam of light from a luminous area is separated into two or more parts by creating partial reflections, the parts being subsequently reunited after traversing different optical paths. Interferometers are capable of precise measurement of very small distances by analysis of interference "fringes". Phase differences between the isolated portions of the original beam of light generate interference "fringes" which bear a unique relationship to the wavelength of light used. Displacement and Doppler shift interferometry are methods used for detecting motion on a surface without mechanical contact which could change the natural motion of the surface.
Spectral analysis includes plasma and emission spectroscopy. Plasma spectroscopy involves using a focused high powered laser to ablate material, thereby convening it into an ionized plasma which has a spectral linewidth unique to the molecular/atomic make-up of the material which is ablated. Emission spectroscopy uses a low to medium powered laser to stimulate a target material to radiate a wavelength of light different from that of the laser.
VISAR (Velocity Interferometer System for Any Reflector) is a specialized Doppler interferometer system that optically measures acceleration, displacement and velocity using coherent, single frequency laser light to illuminate a target that exhibits reflectivity. Reflected light is analyzed using a modified unequal leg Michelson interferometer. VISAR was first developed by Barker and Hollenbach as a means for measuring particle velocities of materials in gas gun experiments. (L. M. Barker and R. E. Hollenbach, "Laser Interferometer for Measuring High Velocities of Any Reflecting Surface," Journal of Applied Physics 43:11, November, 1972). It has been modified and refined over the years since its initial introduction, and it has become the world-wide accepted standard for shock phenomena analysis.
Certain limitations are inherent in traditional laser spectroscopy, VISAR and other Doppler interferometry methods as a result of the equipment having large power and cooling requirements and a sensitive and complex nature. Also, existing VISAR technology requires the use of an open and potentially hazardous laser beam. Consequently, many interferometric applications have been typically restricted to use in controlled laboratory situations. Nevertheless, many times there is a need to locate either the light source or the interferometer, itself, at a distance remote from the target to be analyzed. For example, in instances where explosions or detonations could alter data or cause damage to instruments, it would be desirable to remotely gather data for analysis in real time. Likewise, situations in which a target is either inaccessible or in a location which is hazardous to human beings would necessitate remote data acquisition. Also, there are cases where it may be desirable to perform interferometric analysis while minimizing disturbance to surrounding structures, such as within human or animal tissues.
Existing methods for directing a laser beam to a target require locating the laser and diagnostic apparatus close to the target (within approximately 30 meters) and using mirrors to direct the laser beam to and from the surface being analyzed. This precludes effective use of VISAR, other forms of open-beam interferometry and laser spectroscopy in analysis under conditions wherein the "line of sight" of the laser beam is interrupted, such as through smoke or dust or inside tunnels or enclosed chambers. Also, since the instrumentation involved in the production of the laser beam and collection and analysis of reflected or emitted light is bulky and sensitive, and subject to corruption and degradation by environmental factors, photon analysis as it is practiced using presently available technology is often not well suited to field use or applications in harsh environments.
There is an ongoing need in the field of photon analysis for versatile instruments which are rugged and durable and which exhibit remote sensing capability.